The present invention relates to a unit and a process for cutting ferrous metals, in particular structural steels, in which the metal is locally preheated by a stream of plasma delivered by a plasma torch and the metal is cut by a stream of pressurized oxidizing gas, such as a stream of cutting oxygen, delivered by a delivery nozzle or the like.
At the present time, several processes are known for the automated thermal cutting of metals, said processes having been used for many years on an industrial scale.
By way of examples, mention may be made of oxycutting, plasma cutting and laser cutting, especially for structural steels.
These processes are based on local melting, over the entire thickness, of the material to be cut and on the displacement of the melting front in a path which defines the shape of the cut or kerf that has to be made through the material to be cut.
These various processes cannot actually be considered as competing processes as they are distinguished from one another by different cutting performance and operating and running costs.
Thus, the technique of oxycutting is known for its ability to cut on an industrial scale structural steel thicknesses ranging from 3 mm to 300 mm and to reach, in rarer applications, thicknesses possibly up to 2000 mm.
In this case, although the cost of the cutting tool, that is to say the torch, may be low, an oxycutting process has especially the drawback of being excessively slow overall.
On the other hand, plasma cutting is known for its ability to cut any type of metallic material with a very high productivity.
However, the cost of the cutting tool, namely the assembly consisting of the plasma torch and the current generator, is usually from 30 to 50 times higher than in the previous case, namely in oxycutting.
Moreover, CO2 laser cutting is known to produce excellent cutting quality, particularly over thicknesses of less than 10 mm, that is to say within a thickness range in which the laser process is also productive.
In contrast, the cost of the cutting tool, namely the assembly consisting of the laser head and the CO2 laser source, is, here too, 200 to 300 times higher than that in the case of oxycutting.
More generally, the oxycutting technique is based on the use of the thermal energy generated by the combustion of iron, combined with the kinetic energy of the oxygen jet which allows the oxides produced during said cutting to be expelled from the kerf.
However, the combustion of iron requires the presence of preheating flames to initiate it and then sustain it correctly.
To do this, oxycutting torches are conventionally fitted, at their lower end, with a cutting head or delivery nozzle, generally cylindrical in shape, having a central channel for delivering the cutting oxygen which is surrounded, at a certain distance away, by a ring of channels for delivering a mixture of a combustible gas and an oxidizer which are intended to form a heating or preheating flame peripheral to the central oxygen cutting jet.
An oxycutting operation may be described schematically by a cycle comprising the following steps:
(a) opening, by an operator, of the combustible-gas and oxidizer taps so that these gases are fed into the heating orifices of the cutting head;
(b) ignition of the cutting torch, either manually by means of, for example, a lighter flame presented at the exit of the heating orifices of the cutting head, or automatically, for example with the aid of a piezoelectric quartz crystal for creating a spark which ignites a gas pilot, whose flame thus obtained is directed toward the heating orifices of the cutting head, so as to ignite in turn the heating flame of the torch;
(c) adjustment of the combustible gas and oxidizer flow rates, by means of taps provided on the torch, so as to obtain a flame with the chosen stoichiometric ratio or corresponding to the technical requirements of the torch manufacturer;
(d) presentation of the torch at the required point of initiation on the workpiece to be cut;
(e) local heating of the workpiece to be cut until a sufficient temperature is reached, conventionally about 1300xc2x0 C. in the case of a workpiece made of structural steel, in order for the iron-oxygen reaction to be able to be initiated and sustained;
(f) opening of the cutting oxygen;
(g) drilling of the workpiece over its entire thickness;
(h) movement of the torch by means of the shafts of the cutting machine and execution of the cutting in one or more programmed paths;
(i) end of the cutting operation, stopping the feed of gas to the torch in order to stop the flow of cutting oxygen and the heating or, where appropriate, cutting off the flow of cutting oxygen and continuing the heating in order to move the torch to a new point of initiation.
However, the productivity of oxycutting processes generally suffers from a low rate of propagation of the combustion front of the iron forming part of the composition of the material to be cut and also by the relatively long times to prepare for the actual cutting, that is to say the time to adjust the heating flame and the time for heating the workpiece locally in order to reach the temperature favorable to the iron oxycombustion reaction.
Thus, because of a low heating power density applied to the workpiece, the time needed to raise the material to the required temperature is generally from 5 to 20 seconds and may, in extreme cases, be as long as about 1 minute.
In addition, this heating phase followed by the initiation of the oxycombustion reaction cannot be easily automated because the time needed to reach the correct reaction initiation conditions cannot be accurately predicted.
This is because the factors that can influence this time are, especially, the mass of the workpiece, the thermal conductivity of the grade of material to be heated, the surface state of the material, that is to say for example the possible presence of millscale, grease, paint or another coating on this material, but also other factors associated with the specific heat of the gases used for heating, and their mixing ratio.
In practice, most often the operator carefully monitors the heating operation and manually opens the cutting oxygen when conditions suitable for initiating the oxycombustion reaction seem to him to be achieved.
This practice sometimes leads to ignition xe2x80x9cfailuresxe2x80x9d, that is to say ineffective or imperfect ignition, because the temperature of the material is not high enough, or sometimes, on the other hand, to excessively long heating times, for safety""s sake, in order to be sure that ignition will take place correctly.
Consequently, the problem which arises is to prevent or minimize ineffective or imperfect ignition and to increase the productivity of oxycutting processes by, in particular, reducing the time needed to prepare for the actual cutting operation, and preferably with effective automation of the entire process.
The solution proposed by the present invention relies on coupling an oxycutting process with a process for heating by means of a plasma jet or stream of plasma, and of its operating equipment.
The present invention therefore relates to a process for the plasma oxycutting of at least one metal workpiece containing at least one ferrous metal, in particular iron, in which:
(a) an ignition region of the metal workpiece to be cut is locally preheated by subjecting said ignition region to at least one plasma jet;
(b) at least part of the ignition region at least preheated in step (a) is subjected to at least one stream of oxidizing gas at a pressure of greater than 105 Pa;
(c) at least one drillhole is made over the entire thickness of the workpiece to be cut, in at least part of the ignition region subjected to at least preheating by plasma jet in step (a), by melting and/or combustion of the ferrous material contained in said metal workpiece by the reaction of said ferrous material with said stream of oxidizing gas and/or said plasma jet;
(d) the plasma jet and the stream of oxidizing gas are moved in a cutting path in order to produce at least part, that it to say at least a portion, of a kerf through said workpiece by melting and/or combustion of the ferrous material contained in said metal workpiece by means of the reaction of said ferrous material with at least said stream of oxidizing gas.
Depending on the case, the process according to the invention may comprise one or more of the following characteristics:
the drillhole produced in step (c) is obtained by the reaction of the ferrous material with at least said stream of oxidizing gas. In this first case, it is essentially the stream of oxidizing gas which is used to drill the metal workpiece and the plasma jet serves, on the one hand, only to preheat the ignition region and possibly to obtain the start of melting and/or combustion of the iron contained in the material of which the metal workpiece is composed and, on the other hand, to sustain the oxidizing combustion flux;
the drillhole produced in step (c) is obtained by the reaction of the ferrous material with said plasma jet. In this second case, it is the plasma jet which is used not only to preheat the ignition region but also to drill the metal workpiece by the melting and/or combustion of the iron contained in the material of which the metal workpiece is composed and then serves, as in the first case, to sustain the oxidizing combustion flux;
the ignition region is preheated in step (a) to a temperature of between 1000 xc2x0 C. and 1500 xc2x0 C., preferably from 1200 xc2x0 C. to 1400 xc2x0 C. and even more preferably about 1300 xc2x0 C. to 1350 xc2x0 C.;
the preheating time is between 0.001 and 2 seconds, preferably between 0.01 and 1.5 seconds;
the pressure of the stream of oxidizing gas is set or adjusted depending on the thickness to be cut and/or on the heating energy generated by the plasma jet;
the flow rate of oxidizing gas is greater than 1 l/min and preferably the flow rate of the stream of oxidizing gas is set or adjusted depending on the thickness to be cut and/or the heating energy generated by the plasma jet;
the stream of oxidizing gas is oxygen or a gas mixture containing oxygen, especially air;
the melting and/or combustion of the ferrous material by the stream of oxidizing gas is localized to at least part of the ignition region;
during cutting, each portion of the cutting path is subjected to the plasma jet and to the stream of oxidizing gas, most of the melting and/or combustion of the material in said cutting path being essentially provided by the reaction of iron with the stream of oxidizing gas;
the plasma jet and the stream of oxidizing gas are delivered coaxially or convergently ;
the kerf is produced by moving said plasma jet and said stream of oxidizing gas at an approximately constant cutting rate, preferably at a cutting rate which depends on the thickness to be cut, on the gas flow rate and/or on the gas pressure, for example a cutting rate of about 0.6 m/min for a steel plate having a thickness of 12 mm.
The invention also relates to a plasma-oxycutting unit that can be used to cut a metal workpiece containing at least one ferrous material, in particular iron, comprising at least:
a plasma jet preheating torch of axis (Ztxe2x80x94Zt) for delivering at least one plasma jet and a gas stream delivery nozzle of axis (Zbxe2x80x94Zb) for delivering at least one stream of gas; the axis (Zbxe2x80x94Zb) of said delivery nozzle and the axis (Ztxe2x80x94Zt) of said preheating torch both being directed toward a point of convergence such that the gas jets emanating from the nozzle and from the torch converge on said point of convergence, preferably the point of convergence lying approximately at or near the upper surface of the metal workpiece;
support-frame means supporting said plasma jet preheating torch and/or said delivery nozzle;
movement means for moving, preferably in approximate synchronism, the plasma torch and the delivery nozzle relative to the metal workpiece; and
control means used for controlling at least the movement means and/or at least one operating cycle of the torch, preferably the operating cycles of the torch.
Depending on the case, the unit according to the invention may comprise one or more of the following characteristics:
one or more gas sources;
at least one electric current source;
means for supplying a coolant, for example water;
the plasma torch and the delivery nozzle are coaxial, preferably with noncoincident respective axes, or with convergent axes;
the plasma torch is of the single-flow or multiflow type, especially the dual-flow type;
the plasma torch is of the blown-arc and/or transferred-arc type;
the movement means are motorized;
it includes at least one workpiece support means for supporting and/or holding at least one metal workpiece to be worked;
it furthermore includes means for controlling the relative movements between the torch and/or the nozzle, and the workpiece to be worked;
it also includes means for feeding the workpiece to be worked and/or removing the worked workpiece, that is to say after the workpiece has been worked;
it comprises means for programming the cutting paths, means for programming the paths for transfer from one cutting program to another cutting program and/or means for programming the ignition and/or extinction sequences of the plasma-oxycutting unit.
In other words, according to the present invention, the oxyfuel xe2x80x9cheatingxe2x80x9d means used in conventional oxycutting processes is replaced by a plasma jet xe2x80x9cheatingxe2x80x9d means.
The plasma jet is created by an electric arc established in a stream of plasma gas between a first electrode forming part of the plasma torch and the workpiece to be heated and cut, which forms a second electrode.
For example, the electrode of the torch, or first electrode, is connected to the negative pole of a DC current source and the workpiece to be heated and cut is connected to the positive pole of said source.
Preferably, oxygen or an oxidizing gas having similar iron combustion properties is used for the stream of plasma gas.
An orifice plate or nozzle, vigorously cooled and comprising an orifice through which the stream of plasma passes and from which it is expelled, is placed in the path of the plasma arc between the cathode and the anode, so as to increase the power density deposited on the workpiece to be heated/cut, by means of a constriction of the plasma arc through said orifice.
The power of the plasma arc is adjusted so that the material to be heated/cut is locally and rapidly raised to a temperature close to the melting point, that is to say about 1300 xc2x0 C., without thereby being melted through the depth by the impingement of a plasma jet which would be too energetic.
Next, when the temperature of the ferrous material locally reaches a temperature close to the melting point of said material, an oxygen jet is sent onto the region thus preheated so as to initiate the iron oxycombustion reaction.
As in the case of a conventional oxycutting process, this iron combustion reaction, which is highly exothermic in nature, therefore causes progressive melting and combustion of the material, right through its thickness, with the formation of a kerf by the expulsion of the molten material due to the blowing effect created by the kinetic energy of the pressurized oxygen jet and to do so in a predetermined cutting path corresponding to the movement of the oxycutting torch, preferably at a uniform and appropriate rate.
It will be immediately understood that with a heating means whose temperature may reach 20,000 K in the core of the plasma jet and whose power density is temperature from ambient temperature up to about 1300xc2x0 C. is much shorter than with an oxyfuel flame whose flame temperature is only about 3275 K (in the case of an oxyacetylene flame) and whose power density, at impingement on the workpiece, is only about 2 kW/cm2.
Thus, the necessary heating time, before initiation, is from 5 to 20 seconds with an oxyfuel flame whereas it is reduced to about {fraction (1/10)}th of a second when preheating using a plasma arc according to the present invention.
In addition, under optimum conditions for using the plasma preheating jet, this heating time is not influenced very much by the mass, the grade and the surface state of the ferrous material to be heated and cut, which therefore allows relatively easy automatic management of all the work phases resulting in the final cutting of the workpieces.
The power of the plasma jet may also be varied depending on the work phase; for example, the power may be higher during the heating and initiation phase than in the cutting phase.
To vary this power, all that is required is to control the current source supplying the torch, for example by means of a microprocessor, so that the intensity of the current delivered into the plasma arc is adjusted according to the requirements of the current phase of the cycle.
Likewise, the pressure and/or flow rate of the plasma gas may be adjusted in the same phases.
All these parameter adjustments may be preprogrammed, especially when setting up the cutting program, including the workpiece geometries, the points or moments of initiation and/or completion of the process, the cutting speeds, etc.
This preprogramming may be carried out directly via a computerized numerical control (CNC) tailored to the control of the cutting machine or by any other off-line programming means.
This comparative example illustrates the increase in productivity that may result from the oxycutting process with plasma jet preheating according to the invention over a conventional oxycutting process with no plasma jet preheating.
An identical test piece is cut from a structural steel of the E24 type, having a thickness of 20 mm, by carrying out each of the abovementioned processes.
Each time, the cutting operation consists in cutting 25 disks 20 mm in diameter within these test piece and of one perimeter equal to about 1.5 meters around said 25 disks.
In other words, the cutting operation is composed of the cutting of:
25 disks, i.e. 25 initiations and a cutting length of about 25xc3x970.063 m; and
1 perimeter of the test piece, i.e. 1 initiation and a cutting length of about 1.5 m.
The cutting operation therefore comprises, in total: 26 initiations and a cutting length of about 3 m.
Admittedly, to allow easier comparison, the cutting speed used is the same in the tests for implementing a conventional oxycutting process and in tests for implementing a plasma-oxycutting process according to the present invention: in both cases, the cutting speed is 0.6 m/min.
However, it should be emphasized that, when implementing a plasma-oxycutting process according to the present invention, the heating of the oxygen cutting jet near the plasma jet may, in certain cases, allow higher cutting speeds than in conventional oxycutting, that is to say in the case of an oxyfuel flame.
In addition, another factor liable to increase the cutting speed stems from the heat concentration, upon impingement of the plasma jet on the workpiece, limiting the lateral expansion of the iron combustion region and producing narrower kerf widths than in conventional flame oxycutting.
Furthermore, within the context of the tests carried out here, the total cutting time, excluding the time for transferring from one cutting operation to the next, is the same for both types of processes tested, namely 5 min 13 s.
The results obtained are given in the following table.
From the above table it may be seen that the oxycutting process with plasma preheating according to the invention makes it possible to achieve a time saving of about 5 min 21 s over an oxycutting process according to the prior art.
In other words, a plasma-oxycutting process according to the invention makes it possible to cut twice the number of metal workpieces than a conventional oxycutting process in the same time.